In the design of monolithic integrated circuit (IC) devices using PN junction isolation, lateral PNP transistors are commonly employed. However, since the current gain or Beta of such PNP transistors is intrinsically low and falls off at high frequencies, their use as signal amplifiers is restricted. In particular, IC current mirrors commonly employ PNP transistor current sources. For example in copending application Ser. No. 507,309 filed June 23, 1983, now U.S. Pat No. 4,528,496, by Toyojiro Naokawa and Matsuro Koterasawa a Low voltage IC current supply is described. The teaching in this application, which is assigned to the assignee of the present invention, is incorporated herein by reference. Such current mirros can be made to operate accurately at low voltages and they serve usefully as bias supplies. However, when a current mirror is intended to operate at signal frequencies, the PNP transistor frequency limitations can be significant.
In the conventional PN junction isolated monolithic IC, the NPN transistors display one to two orders of magnitude higher frequency response. This means that where signals are to be handled, NPN transistor construction is greatly preferred.